PROJECT SUMMARY Mammary gland development requires complex interactions between mammary epithelial cells and their surrounding environment. Tissue resident macrophages are present in the mammary gland in close association with developing epithelial structures and within the adipose stroma, and are known to be important for contributing to mammary gland development and maintaining tissue homeostasis. Understanding these mechanisms is critical for determining the consequences of altered macrophage function on loss of homeostasis and promotion of tissue specific disease, such as tumorigenesis. The goal of this project is to delineate the mechanisms that maintain tissue resident macrophages in the mammary gland in a homeostatic state and to determine whether the altering these mechanisms impacts mammary gland development. In preliminary studies, we have developed a novel method for identifying and isolating distinct populations of resident macrophages from the mammary gland, including macrophages associated with epithelial structures and those associated with stromal regions. Based on preliminary studies, we propose that signal transducer and activator of transcription (STAT) pathways are key regulators of resident macrophages in the mammary gland and that deregulation of these pathways results in the formation of macrophages that create a permissive environment for tumorigenesis. Proposed studies will 1) determine the source and localization of tissue resident macrophage populations during mammary gland development, 2) demonstrate the importance of STATs as key transcriptional regulators of tissue resident macrophages in the mammary gland and 3) determine the effects of inflammatory factors on resident macrophage function in the mammary gland. Recent studies have focused on understanding the mechanisms driving tissue resident macrophage function and their contributions to tissue development and homeostasis. However, relatively little is known regarding the mechanisms driving resident macrophage function in the mammary gland. Understanding these mechanisms will provide insights into how alterations in resident macrophage function, such as by local or systemic inflammatory signals, impact epithelial morphogenesis and contribute to cancer risk.